Nitrogen oxide (NOx) sensors are used to measure NOx levels in tailpipe emissions. In particular, NOx sensors are used in exhaust systems of modern diesel and lean-burn petrol engines to monitor tailpipe emissions. In a vehicle, a NOx sensor is typically coupled to a controller area network (CAN) to receive information from other sensors about the operating conditions of the vehicle. The NOx sensor measures NOx levels in tailpipe emissions based on the information about the operating conditions of the vehicle from the other sensors via the CAN.
One difficulty with NOx sensors is testing the NOx sensors, particularly in vehicle repair facilities. Traditionally, testing a NOx sensor in a vehicle includes running the vehicle for a time until the vehicle reaches a stable operating condition and then reading the NOx sensor output via a diagnostic connection of the vehicle's CAN. This allows the NOx sensor to receive information about the operating conditions of the vehicle from the other sensors via the CAN and provide its output via the CAN to the diagnostic connection.
This approach has a number of disadvantages. The time needed to bring the vehicle to the stable operating condition—sometimes up to 40 minutes or more—adds to the overall repair time for the vehicle. A faulty NOx sensor reading via the diagnostic connection of the vehicle's CAN may not be the result of a faulty NOx sensor because the NOx sensor reading via the diagnostic connection of the vehicle's CAN relies on a number of variables, such as the operation of the CAN and the operation of the other sensors coupled to the CAN. These disadvantages make testing of NOx sensors difficult and time-consuming, particularly in the repair facility setting where NOX sensors are tested via the CAN while installed on the vehicle.